1. Field of the Invention
The present invention relates to a guard system for a main switching device of an inverter apparatus, and in particular to a guard system for inverter apparatus having self-cutoff function devices.
2. Description of the Related Art
With the advance of development of various power semiconductor devices in recent years, relatively large-capacity inverter apparatus using so-called self-cutoff switching devices such as GTO's (Gate Turn-Off Thyristors) or power transistors have been brought into use.
In general, there is a strict region of voltages or currents with which such a self-cutoff switching device can be switched safely. Switching operation beyond this region immediately results in permanent destruction of the device.
In an inverter apparatus using such self-cutoff switching devices, therefore, various guard devices are provided. A conventional method is disclosed in Japanese Patent Unexamined Publication No. 59-63983, for example. If the voltage or current of the main circuit of the inverter becomes an abnormal condition and such condition remains in a range recoverable by bringing the condition of the switching devices into the cutoff state, the switching devices of the main circuit of the inverter are simultaneously cutoff to carry out guard function. That is to say, the so-called simultaneous cutoff guard function is operated in the conventional method. If the failure state exceeds the range in which the state can be controlled by means of switching devices, the so-called simultaneous turn-on guard function is operated. That is to say, the switching devices of the main circuit of the inverter are simultaneously turned on to prevent the destruction of the switching devices. Additionally the entire circuit is guarded by cutting off the circuit using a fuse or a breaker.
However, it has been found that consecutive guard operation attempts of these guard functions destroy the switching devices contrary to expectation.